1. Field of the Invention
The present invention relates generally to load lifting and lowering devices and, more particularly, to a lift having a platform which is able to rest on or near the ground and remain substantially level during raising and lowering.
2. State of the Art
A common type of load elevator or lift comprises a substantially rectangular platform connected at one edge to the swingable ends of a laterally spaced apart pair of linkage systems. The other ends of the linkage systems are pivotally connected to a support such as the chassis of a vehicle or a loading dock. A power means, usually hydraulic, is interconnected to the pair of linkage systems for moving the platform between ground level and that of the truck or dock or the like. Each of the linkage systems comprises first and second lateral links in the form of an upper arm and a lower arm. The swingable ends of the linkage systems comprise a first vertical link between the upper and lower arms in the form of a shackle against which the weight of the platform and the load thereon are imposed. The other ends of the linkage systems comprise a second vertical link between the upper and lower arms in the form of the support to which the linkage systems are pivotally connected. In this manner, the upper and lower arms, the shackle and the support define a substantially parallelogram-shaped arrangement for each of the linkage systems.
A problem typically encountered with the above-described linkage systems is that they do not maintain a perfect parallelogram arrangement when moving the platform between a raised and lowered position. Rather, the first vertical link is slightly rotated during translation such that the platform describes an arc as it is being raised or lowered. When located at or near ground level, this causes the outer edge of the platform to be tilted or ramped towards the ground. This may not be suitable for some loads, such as those resting on wheels or other mechanisms allowing for lateral load movement.
One approach to addressing this problem has been to lower the platform a first distance wherein the linkage systems are maintained in the parallelogram arrangement, i.e. without substantially rotating the first vertical link, and then using additional mechanisms to move the platform a second distance into contact with the ground. This two-step translation method enables the platform to have a substantially level orientation through most of its lifting range, but requires a complex lift assembly that is more difficult to manufacture and maintain. Moreover, the platform may still have to be slightly tilted when translating through the second distance in order to reach the ground.
What is needed is a lift that has a platform with a substantially level ride through its entire lifting range and that does not require the use of complex mechanisms for lowering the platform to a location on or near the ground.